"This morning Paul Manafort offered, and I accepted, his resignation from the campaign," the Republican presidential nominee said in a statement.

"I am very appreciative for his great work in helping to get us where we are today, and in particular his work guiding us through the delegate and convention process," Trump added. "Paul is a true professional, and I wish him the greatest success."

Manafort's role in the Trump campaign was marginalized earlier this week with the hiring of Brietbart News chairman Steve Bannon, who is now CEO of the campaign, and the promotion of Kellyanne Conway, who is now the campaign manager.

But his resignation comes as a surprise. Manafort sent a memo to campaign staffers earlier this week announcing the new hires and saying he would remain in his role as chief strategist.

"I remain the campaign chairman and chief strategist, providing the big-picture, long-range campaign vision and working with all of you to implement our strategy that will guide us to victory in November," he said in the memo.

Manafort's role in the Trump campaign was short-lived. He has been effectively running the campaign since June, when Corey Lewandowski was fired as campaign manager.

Kevin Cirilli, a political reporter for Bloomberg, tweeted that Manafort had been "getting 'the look' from Trump," meaning the candidate "wasn't accepting his advice anymore."

"Staff, surrogates, stunned," Cirilli wrote on Twitter. "Manafort was doing work right up until yesterday."

A close associate of Manafort's told The Washington Post that Manafort was leaving the campaign on good terms and would remain a close ally and outside confidant.

Trump aides told The Post that Manafort wanted to give the new campaign hires room to develop their own strategy.

Manafort, an experienced political operative, had been initially brought on during the primary season to help run the campaign's delegate strategy.

But despite his blowout victory in the primaries, Trump hasn't been doing so well in general election polls. He has been trailing the Democratic nominee, Hillary Clinton, in nearly every national poll for months. Trump surpassed Clinton briefly after the Republican National Convention in July, but Clinton overtook him once more after the Democrats held their convention the following week.

Manafort's resignation comes amid media reports about his ties to Russia and former Ukrainian President Viktor Yanukovych.

A Republican strategist close to Manafort told Post reporter Robert Costa that Manafort was undone in part by the reports of his work for pro-Russian forces in Ukraine.

Russia annexed the Crimean peninsula in Ukraine in 2014, a move that the international community widely regards as illegal. Trump, however, suggested in an interview last month that he supported Russia's invasion of Crimea. And language in the Republican Party platform that referenced supporting Ukrainian nationalists was removed.